1. Field of the Invention
This invention relates to an apparatus for transmitting data over a digital transmission system having discrete data channels with a predetermined base bit-rate lower than the bit-rate of the data to be transmitted. More particularly, the invention provides a method and apparatus for allocating a number of base rate channels to a virtual channel capable of carrying signals having a bit-rate higher than the base rate.
2. Description of the Prior Art
One data transmission system to which the invention is especially applicable is the Integrated Services Digital Network (ISDN) for which the standards have been defined by the International Telegraph and Telephone Consultative Committee (CCITT). ISDN makes use of a 2.048 megabyte per second primary rate TDM channel onto which are time-division-multiplexed 32 sub-channels (DS0). Each subchannel has a base bit-rate of 64 Kb per second. The primary rate TDM channel carries 8000 frames per second, each divided into 32 time slots carrying one byte (8 bits) of data from each sub-channel. The first time slot in each frame is used to identify the start of the frame, and another time slot, which constitutes the D channel, carries routing instructions. The remaining 30 slots are available for carrying data, normally as 30 discrete channels.
The primary rate TDM channel might, for example, be used to connect a private branch exchange (PBX) to the public telephone network. A single primary rate channel will therefore give the subscriber access to thirty 64 Kbps base rate channels. Situations often arise, such as in the transmission of image and video signals, or a high volume of computer data, where it is desirable for the subscriber to transmit the data at a rate higher than the base rate. For example, it would be desirable to have the capability of sending a 128 Kbps bit stream over two parallel 64 Kbps base rate channels. Unfortunately, because of the switching requirements and propagation characteristics of the public network, a channel which occupies a particular time slot in any given frame of the transmitted signal does not necessarily occupy the same time slot at the far end. The base rate channels are subject to different delays through the network. As a result, if the channels are merely reassembled sequentially at the far end, the transmitted data is scrambled and unusable.
International Patent Application No. WO 85/04300 describes a system wherein prior to data transmission synchronization signals are transmitted along each of the base channels to determine the delays applicable to each channel. A reframer unit then takes into account these delays to reassemble the transmitted data in the correct order. The problem with this system is that once the virtual channel has been established it cannot be changed without being completely reset. Furthermore if the delays for the various channels change during transmission, the data becomes unusable. The system cannot therefore be regarded as reliable.
U.S. Pat. No. 4,805,167 describes a system for providing a variable data rate aggregate channel. In this system marker signals are sent on the base rate channels to specify the order of transmission of the sub-signals to as to ensure correct reassembly at the far end. One problem with this system is that it can only be used for packet transmission since it requires there to be idle time slots in the data channels to carry the marker signals. It cannot therefore be used with continuous signals, such as video signals because there are no vacant slots in which to insert the marker signals. Also, since the marker signals can only be inserted when idle time slots are available, it does not permit the delay characteristics of the network to be continually monitored.
An object of the invention is to provide an improved transmission system which does not depend on vacant slots being available in the data stream and which permits continuous monitoring of the channel delays.